The invention relates to a cathode ray tube comprising in an evacuated envelope an electron gun for generating an electron beam which is focused onto a target by means of at least one accelerating electron lens which, viewed in the direction of propagation of the electron beam, comprises a first and a second electrode placed coaxially around the electron beam.
Such cathode ray tubes are used, for example, as a black-and-white or colour display tube for television, as a television camera tube, as a projection television display tube, as an oscilloscope tube or as a tube for displaying digits or characters. This latter type of tube is sometimes termed a DGD tube (data graphic display tube).
Such a cathode ray tube is known, for example, from Netherlands Patent Application No. 7812540 laid open to public inspection and corresponding to U.S. Pat. No. 4,310,776. The electron gun system of a colour display tube described in this Application comprises three electron guns situated with their axes in one plane. The second electrode of the accelerating electron lens of each gun present on the side of the display screen is connected to a common centring sleeve. It is also possible that in addition the first electrodes of the accelerating electron lens form a common component. This is the case, for example, in a so-called integrated electron gun which is also described in the Netherlands Patent Application No. 7812540.
The dimensions of the spot are very important in such tubes. In fact they determine the definition of the displayed or recorded television picture. There are three contributions to the spot dimensions, namely: the contribution as a result of the differences in thermal emanating rates and angles of the electrons emanating from the emissive surface of the cathode, the contributions of the space charge of the beam and the spherical aberration of the electron lenses used. This latter contribution is caused in that electron lenses do not ideally focus the electron beam. In general, electrons which form part of the electron beam and which enter an electron lens farther away from the optical axis of the lens are deflected more strongly by the lens than electrons which enter the lens nearer along the axis. This is termed positive spherical aberration. The spot dimensions increase by the third power of the beam parameters, for example, the angular aperture of the diameter of the incident electron beam. Spherical aberration is therefore sometimes termed a third order error. Long ago (W. Glaser, Grundlagen der Elektronenoptik" "Principles of Electron Optics", Springer Verlag, Vienna 1952) it was demonstrated that in the case of rotationally symmetrical electron lenses in which the potential beyond the optical axis is fixed, for example, by means of metal cylinders, a positive spherical aberration always occurs.